1. Field of the Invention:
This invention relates to a polymer solid electrolyte and to a battery which utilizes a polymer solid electrolyte.
2. Description of the Prior Art:
A great deal of research work has been conducted into the possibility of using polymer solid electrolytes in batteries, displays, etc. Referring by way of example to the use of a polymer solid electrolyte in a lithium battery, it is interposed between a positive electrode and a negative electrode. There have been proposed a variety of series of materials for polymer solid electrolytes, including a series of polyethylene oxides, a series of polyphosphazenes and a series of polyamino acids.
A solid electrolyte battery has a number of advantages. For example, it is leakage-free, it has a high energy density, it is inexpensive, and it is easy to form in a layer-built structure. The material which has so far been tested more extensively than any other material is the series of linear polyethers. The materials of this series, however, have a melting point of about 60.degree. C. and are used only at a temperature above their melting point, since at a temperature below their melting point, the molten salt undergoes crystallization and fails to exhibit a satisfactorily high ionic conductivity.
A series of crosslinked polyethers have been proposed to improve the linear polyethers. These materials have no melting point. Even at or around ordinary room tempeature, therefore, the molten salt hardly undergoes any crystallization, but shows a relatively good ionic conductivity. Trifunctional polyethers which are crosslinked with diisocyanates, such as tolylene 2,4-diisocyanate and hexamethylene diisocyanate, are, among others, suitable for use on an industrial basis. They are easy to produce and a thin film thereof is easy to form. They are expected to be particularly useful as an electrolyte for a rechargeable battery having a lithium anode.
A number of problems, however, arises from the use of any such crosslinked material in a common type of battery in which an intercalation-type metal compound is used as a cathode, and lithium as an anode. The first problem resides in a reduction of performance which occurs to the battery during its storage. The active hydrogen in the polymer solid electrolyte reacts with lithium during storage of the battery and thereby raises its internal resistance, resulting in a lowering of its capacity. The active hydrogen means hydrogen in an -OH group or in an --NH group, which reacts directly with an alkali or alkaline earth metal, or their ions. If a polyether crosslinked with isocyanate is used as the electrolyte, it means hydrogen in the urethane bond NHCOO.
The second problem resides in the poor charging and discharging repeatability of the battery. This is apparently due to the fact that the polymer solid electrolyte is a bi-ionic conductor. If a common type of battery as hereinabove mentioned is discharged, anions (lithium ions) move from the anode to the cathode and lithium is intercalated in the cathode. The anions in the polymer electrolyte (e.g., perchloric acid ions in a battery containing molten lithium perchlorate) move toward the anode. However, the anode does not accept the anions, but the anions cause polarization in the electrolyte layer in the vicinity of the anode. If the battery is, then, charged, the cations which have been intercalated in the cathode can reversibly move to the anode through the electrolyte, but the anions which have polarized in the vicinity of the anode can return to their place of origin only very slowly. As a result, the ions in the polymer have a low degree of reversibility leading to a great reduction in battery capacity with repeated charging and discharging. Attempts have, therefore, been made to synthesize a single-ionic mobile polymer solid electrolyte in which only the cations are mobile, while the anions are fixed to the polymer chain. They have, however, been unsuccessful because of a great reduction of ionic conductivity, or for other reasons.